The present invention maximizes the profitability of a solvent refining unit. Some solvent refining units have tried to maximize the yield for other solvent refining systems such as described in U.S. Pat. No. 3,666,931 and U.S. Pat. No. 3,718,809. These control systems use elaborate schemes to obtain optimum balance of extract oil versus refined oil or relate the quality of the refined oil to the viscosity index using a predicted characteristic constant of the charge oil A and utilizes the economic value of the charge oil, the refined oil and the extract oil to determine the optimum.